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What do galaxies orbit?

 
 
oralloy
 
  1  
Reply Tue 26 Sep, 2006 03:54 am
Re: What do galaxies orbit?
aperson wrote:
I'm not a scientist, but I was just wondering, if a moon orbits a planet, a planet orbits a star, and a star orbits a black hole, what do the black holes in the centre of galaxies orbit in the clusters of galaxies, and what do the objects in the middle of clusters of galaxies orbit. I know that they orbit the centre of the universe, but what exactly causes them to do this?


There is no center of the universe. I imagine that smaller clusters of galaxies orbit larger clusters of galaxies.



aperson wrote:
Stars can't just orbit nothing, unless they're floating on their own out there in the middle of intergalactic space, which I don't think is possible.


Isn't that what irregular galaxies are?



aperson wrote:
Alrighty.

I thought that galaxies or clusters or whatever orbited the centre of the mass of the universe?


No such thing as the center of the mass of the universe.
0 Replies
 
oralloy
 
  1  
Reply Thu 28 Sep, 2006 12:19 pm
Re: What do galaxies orbit?
oralloy wrote:
aperson wrote:
I'm not a scientist, but I was just wondering, if a moon orbits a planet, a planet orbits a star, and a star orbits a black hole, what do the black holes in the centre of galaxies orbit in the clusters of galaxies, and what do the objects in the middle of clusters of galaxies orbit. I know that they orbit the centre of the universe, but what exactly causes them to do this?


There is no center of the universe. I imagine that smaller clusters of galaxies orbit larger clusters of galaxies.


The Milky Way is part of the Local Group of galaxies. The Local Group is at the edge of the Virgo Supercluster. At the heart of the Virgo Supercluster is a really huge cluster of galaxies known as the Virgo Cluster. At the heart of the Virgo Cluster is a really massive galaxy (one of the largest galaxies known) by the name of M87. At the heart of M87 is one of the largest black holes known. That black hole is the center of our supercluster, and if there is any point that everything in the supercluster orbits, that is it.

That black hole is shooting an extremely powerful beam of energy from out of M87.

http://imgsrc.hubblesite.org/hu/db/2000/20/images/a/formats/large_web.jpg

Full resolution: http://imgsrc.hubblesite.org/hu/db/2000/20/images/a/formats/full_jpg.jpg

More: http://hubblesite.org/newscenter/newsdesk/archive/releases/2000/20/image/a
0 Replies
 
stuh505
 
  1  
Reply Wed 4 Oct, 2006 04:38 pm
Re: What do galaxies orbit?
Quote:
No such thing as the center of the mass of the universe.


I see that there is no central point of expansion, because expansion happens uniformly, but I don't see that there is no center of mass. If you deny that there is a center of mass for the universe, you must also deny that there is a center of mass for a baseball.
0 Replies
 
oralloy
 
  1  
Reply Wed 4 Oct, 2006 06:28 pm
Re: What do galaxies orbit?
stuh505 wrote:
Quote:
No such thing as the center of the mass of the universe.


I see that there is no central point of expansion, because expansion happens uniformly, but I don't see that there is no center of mass. If you deny that there is a center of mass for the universe, you must also deny that there is a center of mass for a baseball.


A baseball has an edge and a center.

The universe has no edge and no center.
0 Replies
 
stuh505
 
  1  
Reply Wed 4 Oct, 2006 08:35 pm
Quote:
A baseball has an edge and a center.

The universe has no edge and no center.


1) You cannot use the inverse of a statement as evidence for disproving the same statement -- "...because it has no center" cannot be used as evidence for why there is no center!!

2) A baseball is a set of particles. A planet is a set of particles. A nebula is a set of particles. A galaxy is a set of particles. The universe is a set of particles. Particles have positions (approximately). An edge can be defined as the convex hull containing a set of positions. Tell me, exactly how many particles is required before there is a fundamental impossibility of deriving a convex hull?
0 Replies
 
rosborne979
 
  1  
Reply Wed 4 Oct, 2006 08:45 pm
Re: What do galaxies orbit?
stuh505 wrote:
Quote:
No such thing as the center of the mass of the universe.


I see that there is no central point of expansion, because expansion happens uniformly, but I don't see that there is no center of mass. If you deny that there is a center of mass for the universe, you must also deny that there is a center of mass for a baseball.


Stuh,

There is no center of the Universe. 'Center' is a concept derived from a relative topology which allows for a 'center'.

The Universe has no center of mass, for the same reason it has no topological center.

There is no 'center' to the surface of a sphere either. And if dots (like points of mass) were painted on the surface of a sphere in an evenly distributed manner (as matter is distrubuted in the Universe (refer to COBE and WMAP), there would be no center to the density of the dots either.
0 Replies
 
oralloy
 
  1  
Reply Wed 4 Oct, 2006 09:34 pm
stuh505 wrote:
An edge can be defined as the convex hull containing a set of positions. Tell me, exactly how many particles is required before there is a fundamental impossibility of deriving a convex hull?


An infinite number of particles are required.

The universe has an infinite number of particles.
0 Replies
 
stuh505
 
  1  
Reply Wed 4 Oct, 2006 09:54 pm
Quote:
The universe has an infinite number of particles.


That's just not true.

It's also not part of the typical argument for not having a center, I think you are just making things up now because you don't really know why there is no center, but you've heard experts say it, so you're saying it too.

Quote:
There is no 'center' to the surface of a sphere either. And if dots (like points of mass) were painted on the surface of a sphere in an evenly distributed manner (as matter is distrubuted in the Universe (refer to COBE and WMAP), there would be no center to the density of the dots either.


1) According to this argument, then, no object truly has a center of mass, because the COM of any 2 points on a sphere will be inside the sphere.

2) My astronomy teacher also said that the matter was on the surface of a sphere. In fact he was quite convinced of it...but he didn't know why.

And I hear it over and over, and there are websites that say it too -- and if it is true, then I agree that there is no center of mass. But why do people think this?

Here's one interpretation I can think of:

All matter started on the surface of a small sphere. The sphere's radius grows at a constant rate, causing all particles in the sphere to expand at an accelerating rate. But the surface of a sphere is 2 dimensional, and our space is at least 3 dimensional, so we have to introduce another dimension to the expanding spherical surface model.

However, if that is the case, then tha argument does not hold up, because the 2 dimensions that define the position on the sphere surface and the 3rd dimension that defines the sphere, can be plotted onto a standard cartesian axis and a COM can be found in conventional ways, and that point would still be on the surface of the hypersphere.
0 Replies
 
oralloy
 
  1  
Reply Wed 4 Oct, 2006 10:27 pm
stuh505 wrote:
Oralloy wrote:
The universe has an infinite number of particles.


That's just not true.


Yes it is.



stuh505 wrote:
I think you are just making things up now because you don't really know why there is no center, but you've heard experts say it, so you're saying it too.


Nope. I am telling you the actual reason there is no center of the universe.
0 Replies
 
stuh505
 
  1  
Reply Wed 4 Oct, 2006 10:33 pm
Quote:
stuh505 wrote:
Oralloy wrote:
The universe has an infinite number of particles.


That's just not true.


Yes it is.


We don't know that. It's likely not.
0 Replies
 
oralloy
 
  1  
Reply Wed 4 Oct, 2006 11:25 pm
stuh505 wrote:
oralloy wrote:
stuh505 wrote:
oralloy wrote:
The universe has an infinite number of particles.


That's just not true.


Yes it is.


We don't know that.


Yes we do.



stuh505 wrote:
It's likely not.


The science says otherwise.
0 Replies
 
oralloy
 
  1  
Reply Wed 4 Oct, 2006 11:39 pm
See if this helps:

http://cmb.phys.cwru.edu/boomerang/papers/debernardis00.pdf
0 Replies
 
timberlandko
 
  1  
Reply Thu 5 Oct, 2006 01:40 am
For there to be an infinite number of particles within the universe entails the volume of the universe be infinite. If that volume be infinite, there would be no horizon. If there be no horizon, there would be no means by which to deduce the singularity from which the observable universe emerged. Were the universe to be infinite, time and space could have no meaning.
0 Replies
 
spendius
 
  1  
Reply Thu 5 Oct, 2006 02:46 am
Do they not have no meaning except in men's minds?
0 Replies
 
oralloy
 
  1  
Reply Thu 5 Oct, 2006 07:50 am
timberlandko wrote:
For there to be an infinite number of particles within the universe entails the volume of the universe be infinite. If that volume be infinite, there would be no horizon.


And that is the case with the universe.



timberlandko wrote:
If there be no horizon, there would be no means by which to deduce the singularity from which the observable universe emerged.


We could observe the microwave background radiation.



timberlandko wrote:
Were the universe to be infinite, time and space could have no meaning.


It seems to me to have meaning.
0 Replies
 
stuh505
 
  1  
Reply Thu 5 Oct, 2006 08:24 am


Oralloy,

This paper is about finding the density of the universe.

D = M/V

Therefore the authors consider M to be a fixed, finite number.

Thank you for disproving your own claim with your own evidence.
0 Replies
 
timberlandko
 
  1  
Reply Thu 5 Oct, 2006 08:53 am
oralloy wrote:
timberlandko wrote:
For there to be an infinite number of particles within the universe entails the volume of the universe be infinite. If that volume be infinite, there would be no horizon.


And that is the case with the universe.

That you believe it so is immaterial to the fact that the available evidence indicates otherwise.



Quote:
timberlandko wrote:
If there be no horizon, there would be no means by which to deduce the singularity from which the observable universe emerged.


We could observe the microwave background radiation.

Without a horizon there would be no cosmic background radiation; there would be no background, there would be nothing from which it might have derived. It is the observable remnant of one edge of the horizon; the Big Bang..



Quote:
timberlandko wrote:
Were the universe to be infinite, time and space could have no meaning.


It seems to me to have meaning.

Apparently some things mean differently to you than to others - not that there's anything wrong with that.


BTW - I noticed something about the paper to which you linked; while neither the word "infinite" nor the word "infinity" occur in it, it does describe a horizon; the age of the universe.
0 Replies
 
oralloy
 
  1  
Reply Thu 5 Oct, 2006 11:05 am
stuh505 wrote:
Therefore the authors consider M to be a fixed, finite number.


The authors established that the universe is infinite, nothing finite about it.
0 Replies
 
oralloy
 
  1  
Reply Thu 5 Oct, 2006 11:06 am
timberlandko wrote:
oralloy wrote:
timberlandko wrote:
For there to be an infinite number of particles within the universe entails the volume of the universe be infinite. If that volume be infinite, there would be no horizon.


And that is the case with the universe.

That you believe it so is immaterial to the fact that the available evidence indicates otherwise.


The evidence, which is conclusive, indicates that the universe goes on forever.



timberlandko wrote:
oralloy wrote:
timberlandko wrote:
If there be no horizon, there would be no means by which to deduce the singularity from which the observable universe emerged.


We could observe the microwave background radiation.

Without a horizon there would be no cosmic background radiation; there would be no background, there would be nothing from which it might have derived. It is the observable remnant of one edge of the horizon; the Big Bang..


That is incorrect. The background radiation is the edge of nothing. It is only the light that was given off when hydrogen and helium atoms formed.

The lack of an edge to the universe does not prevent this light from reaching us.



timberlandko wrote:
BTW - I noticed something about the paper to which you linked; while neither the word "infinite" nor the word "infinity" occur in it, it does describe a horizon; the age of the universe.


True, there is a temporal edge -- a beginning before which nothing existed. But that is something quite different than a spatial edge.

The key to look for in the article is the term "flat" or "open" as opposed to "closed".

A closed universe is finite, while a flat or open universe is infinite.

The studies of the background radiation indicate quite conclusively that the universe is "flat".
0 Replies
 
oralloy
 
  1  
Reply Thu 5 Oct, 2006 11:20 am
oralloy wrote:
A closed universe is finite,


Note: Even if the universe did happen to be closed, there wouldn't be an edge to it. Space would just loop around and repeat itself.
0 Replies
 
 

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